NSF/EAR-BSF: Resolving slow slip transients before and after the 2011 Tohoku-oki earthquake with geodesy and seismicity

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Tectonic plate motions produce stress accumulation at convergent plate boundaries, which is released by slip on faults. Some faults slip rapidly during earthquakes, generating waves, traditionally analyzed by seismologists. Other faults slip at much slower rates and aseismically, without generating waves. An episode during which the fault slips at rates that are about 100 times the long-term tectonic loading rate for periods of up to several weeks is commonly referred to as a slow slip event (SSE). SSEs are often accompanied by weak seismic activity and analyzing them requires data from sensitive geodetic and seismic networks. SSEs are thought to play an important role in the initiation of damaging earthquakes, and it is therefore important to elucidate the physics that controls their behavior. Advances in geodetic instrumentation have enabled the detection of SSEs occurring before several large earthquakes, among which the March 11, 2011 M9.0 Tohoku-oki earthquake is the best-recorded event in history. The spatiotemporal seismo-geodetic network coverage of northeast Japan provides a unique opportunity to study the complex activity leading up to this megathrust earthquake and in its aftermath. Precursory observations reported thus far are very diverse, ranging from a decadal slip acceleration observed by GPS to a final foreshock sequence two days before the main shock. This project will develop a more precise picture of seismic and aseismic slip interactions in the decades before the Tohoku-oki earthquake. The researchers on this project have developed an innovative method which enables the detection of low-amplitude SSEs using geodetic and seismicity data. The results from this research will inform efforts to improve short-term earthquake hazard assessments in all convergent plate boundaries. This project is an international collaboration with researchers in Japan and with a new faculty member at the Tel Aviv University in Israel, through the binational NSF-BSF program. This project aims to analyze the details of aseismic transient slip in the decades leading up to, as well as following the great March 11, 2011 M9.0 Tohoku-oki megathrust earthquake. By using a geodetic matched filter technique, we seek to detect lower-amplitude transient events in geodetic time series than the ones observed with previously developed techniques. The researchers will use existing and new methods to simultaneously analyze geodetic and seismological observations, as they are likely to provide complementary information about a common physical phenomenon. The methodology will be applied first to the Nankai trough, where many slow slip events have already been described, as a calibration and as an experimentation in combining GPS and tilt-meter data in a novel detection technique. Secondly, they will search for transient activities in the Tohoku area, with a focus on two areas, offshore Sanriku and beneath the Boso Peninsula. Offshore Sanriku, repeating earthquakes have shown oscillations in slip rates that still have to be captured by geodetic observation to determine their spatial and temporal extent. Beneath the Boso Peninsula, where a sequence of slow slip events with recurrence intervals disturbed by the Tohoku-oki earthquake is going on, seismicity and geodetic recordings will be integrated to detect lower-amplitude transient signals and to improve the resolution of existing slip models. A primary goal will be to resolve the impact of the Tohoku earthquake stress perturbation on the cycle and dynamics of the slow slip events. Finally, the researchers will develop a comprehensive model at the scale of northeast Japan of the seismic and aseismic cascade of events leading up to the M9.0 Tohoku-oki earthquake.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.